Chopra Nagesh, Kannankeril Prince J, Yang Tao, Hlaing Thinn, Holinstat Izabela, Ettensohn Kristen, Pfeifer Karl, Akin Brandy, Jones Larry R, Franzini-Armstrong Clara, Knollmann Björn C
Oates Institute for Experimental Therapeutics, and Division of Clinical Pharmacology, Vanderbilt University Medical Center, 1265 Medical Research Building IV, Nashville, TN 37232-0575, USA.
Circ Res. 2007 Sep 14;101(6):617-26. doi: 10.1161/CIRCRESAHA.107.157552. Epub 2007 Jul 26.
Cardiac calsequestrin-null mice (Casq2-/-) display catecholaminergic ventricular tachycardia akin to humans with CASQ2 mutations. However, the specific contribution of Casq2 deficiency to the arrhythmia phenotype is difficult to assess because Casq2-/- mice also show significant reductions in the sarcoplasmic reticulum (SR) proteins junctin and triadin-1 and increased SR volume. Furthermore, it remains unknown whether Casq2 regulates SR Ca2+ release directly or indirectly by buffering SR luminal Ca2+. To address both questions, we examined heterozygous (Casq2+/-) mice, which have a 25% reduction in Casq2 but no significant decrease in other SR proteins. Casq2+/- mice (n=35) challenged with isoproterenol displayed 3-fold higher rates of ventricular ectopy than Casq2+/+ mice (n=31; P<0.05). Programmed stimulation induced significantly more ventricular tachycardia in Casq2+/- mice than in Casq2+/+ mice. Field-stimulated Ca2+ transients, cell shortening, L-type Ca2+ current, and SR volume were not significantly different in Casq2+/- and Casq2+/+ myocytes. However, in the presence of isoproterenol, SR Ca2+ leak was significantly increased in Casq2+/- myocytes (Casq2+/- 0.18+/-0.02 F(ratio) versus Casq2+/+ 0.11+/-0.01 F(ratio), n=57, 60; P<0.01), resulting in a significantly higher rate of spontaneous SR Ca2+ releases and triggered beats. SR luminal Ca2+ measured using Mag-Fura-2 was not altered by Casq2 reduction. As a result, the relationship between SR Ca2+ leak and SR luminal Ca2+ was significantly different between Casq2+/- and Casq2+/+ myocytes (P<0.01). Thus, even modest reductions in Casq2 increase SR Ca2+ leak and cause ventricular tachycardia susceptibility under stress. The underlying mechanism is likely the direct regulation of SR Ca2+ release channels by Casq2 rather than altered luminal Ca2+.
心脏肌集钙蛋白基因敲除小鼠（Casq2 - / - ）表现出类似于携带CASQ2突变的人类的儿茶酚胺能性室性心动过速。然而，由于Casq2 - / - 小鼠的肌浆网（SR）蛋白连接蛋白和三联蛋白 - 1也显著减少且SR体积增加，因此难以评估Casq2缺乏对心律失常表型的具体贡献。此外，Casq2是直接还是通过缓冲SR腔Ca2 + 间接调节SR Ca2 + 释放仍不清楚。为了解决这两个问题，我们研究了杂合子（Casq2 + / - ）小鼠，其Casq2减少25％，但其他SR蛋白没有显著减少。用异丙肾上腺素刺激的Casq2 + / - 小鼠（n = 35）的室性早搏发生率比Casq2 + / + 小鼠（n = 31；P < 0.05）高3倍。程序刺激在Casq2 + / - 小鼠中诱导的室性心动过速明显多于Casq2 + / + 小鼠。在Casq2 + / - 和Casq2 + / + 心肌细胞中，场刺激的Ca2 + 瞬变、细胞缩短、L型Ca2 + 电流和SR体积没有显著差异。然而，在异丙肾上腺素存在下，Casq2 + / - 心肌细胞中的SR Ca2 + 泄漏显著增加（Casq2 + / - 为0.18±0.02 F（比率），而Casq2 + / + 为0.11±0.01 F（比率），n = 57,60；P < 0.01），导致自发SR Ca2 + 释放和触发搏动的发生率显著更高。使用Mag - Fura - 2测量的SR腔Ca2 + 不受Casq2减少的影响。因此，Casq2 + / - 和Casq2 + / + 心肌细胞之间SR Ca2 + 泄漏与SR腔Ca2 + 之间的关系显著不同（P < 0.01）。因此，即使Casq2适度减少也会增加SR Ca2 + 泄漏，并在应激状态下导致室性心动过速易感性。潜在机制可能是Casq2直接调节SR Ca2 + 释放通道，而不是改变腔Ca2 + 。
